Vehicles with electrically variable transmissions are commonly driven in part by the transmission including the associated battery and motor/generators. Excess energy from vehicle operation is stored in the battery. The large amounts of kinetic energy from braking are converted to electrical energy and may be used to regenerate the battery charge. However, if the battery has a sufficient state of charge then the excess energy from vehicle braking must be dissipated. Typically, for large vehicles this energy is dissipated through engine/driveline retarder braking and the vehicle service brakes, also known as exhaust braking. The engine retard braking may be coordinated by the vehicle and is not necessarily operator dependent.
In addition to providing power to propel the vehicle, the engine and battery system provide an auxiliary power for various vehicle functions. One of these functions is powering the engine fan. For heavy duty vehicles in particular the engine fan represents a significant parasitic load on the power train which is most cases is provided directly from fuel supplied to the engine. For example, the engine cooling fan may draw from twenty to fifty horsepower when fully engaged.